The disclosure of Japanese Patent Application No. 2002-039005 filed on Feb. 15, 2002 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an antenna system capable of transmitting and receiving radio waves with high sensitivity by switching antenna elements, and more particularly to an antenna system which changes its directivity by switching power supply elements through which power is supplied to the antenna elements by means of a power supply switching device so as to maintain sufficient sensitivity. The invention may be employed as an antenna system for, for example, in-vehicle radio systems, in-vehicle TV signal receiving systems, and vehicle-to-vehicle communication systems.
2. Description of Related Art
A known antenna system mounted on a motor vehicle is shown in FIG. 10. This antenna system is an in-vehicle FM/TV antenna system, which includes four antenna elements 1a, 1b, 1c, and 1d formed or arranged within rear-side glass windows 2, RF amplifiers 3, 4 and coaxial cables 5 provided in power supply portions for respective pairs of the antenna elements 1a, 1b and 1c, 1d, and a receiver 6 including an antenna element switching device. With the antenna system thus constructed, radio waves are received by the antenna elements 1a, 1b, 1c, and 1d that are disposed separated from each other, and the thus received radio waves are amplified by the RF amplifiers 3, 4 that are also disposed separated from each other. The amplified radio waves are then converged while being supplied to the receiver 6 via the coaxial cables 5. Here, the receiver 6 is adapted to select one of the antenna elements which is supplying the highest level of power. Thus, the antenna system is able to receive radio waves while maintaining high sensitivity, such as airwaves, by selecting the most appropriate antenna element. Also, the antenna system constructed as described above can be installed in a motor vehicle without its antenna protruding out from the vehicle body.
As shown in FIG. 10, the antenna elements 1a, 1b (1c, 1d) are arranged close to each other in the same direction. With this arrangement, however, when switching the antenna elements, the directivity of the antenna system only changes slightly. As shown in FIGS. 11a and 11b, it hardly changes. Also, since the sensitivity of the antenna system changes as the vehicle changes its direction, it may happen, depending on the direction of the vehicle, that receiving of radio waves becomes unstable. In addition, since the antenna system essentially includes two RF amplifiers 3, 4 arranged in right and left sides respectively and four coaxial cables 5, this may cause an increase in the cost of the antenna system owing to complicated work required for assembling such parts, high cost for the many parts, and so on.
It is therefore an object of this invention to provide an antenna system which includes at least one antenna element, a plurality of power supply elements, and a power supply switching device, and which is capable of transmitting and receiving radio waves while maintaining sufficient sensitivity by changing the directivity by switching the power supply elements.
To achieve the above object, an antenna system according to one aspect of the invention is provided as an antenna system for emitting/receiving radio waves carrying image signals, audio signals, and/or data signals, and is mainly constituted by at least one antenna element, a plurality of power supply elements through each of which, power is supplied to the antenna element, a power supply switching device, and a power supply cable connected to the power supply switching device. The power supply switching device is operable to switch the power supply elements to connect a selected one or more of the antenna elements to the power supply cable. In the antenna system, a single antenna element or a plurality of antenna elements may be provided. That is, when a plurality of antenna elements are provided, the switches of the power supply switching device are turned on/off to select a specific one or more of the antenna elements via a corresponding one or more of the power supply elements. When a single antenna element is provided, the switches are turned on/off to select a specific one or more of a plurality of routes which are provided by the antenna element and the power supply elements depending on their arrangement and which serve as antenna elements.
In conventional antenna systems, such power supply connection switching is carried out between or among different sets of an antenna element, an amplifier or amplifiers connected to the antenna element, and a power supply cable, while the antenna system according to this invention merely switches the power supply elements. Therefore, unlike such a conventional antenna system, requiring a plurality of power supply cables and amplifies, the antenna system of this invention requires only a single power supply cable and amplifier, thus assuring easier work for installing the antenna system in, for example, a vehicle, and reducing its cost.
In the invention, xe2x80x9cloopxe2x80x9d and xe2x80x9cloop-shapedxe2x80x9d represent not only the shape of a closed loop, but also a loop-like shape which may have one or more portions missing.